1. Field of the Invention
The present invention relates, in general, to systems and methods for forming thin film structures on conductive surfaces, for example, printed circuit boards (PCBs), microelectromechanical (MEM) devices, sensors, and other structures.
2. Description of the Prior Art
As electronic circuit boards become crowded with numerous discrete components, additional advances will require utilization of the inner spaces of electronics packaging for embedded component placement. The embedding of the passive components inside packaging, circuit boards, and interconnections is starting to emerge in the industry as the process effectively combines manufacturing, component packaging and component assembly into a single manufacturing process flow. This embedded technology can thus achieve size reduction, increased functionality, improved high frequency performance, cost savings and increased reliability.
Typically, discrete chip resistors or discrete chip capacitors have been frequently mounted on most printed circuit boards (PCB), but, recently, PCBs are developing in which passive components, such as resistors or capacitors, are embedded.
A technology for fabricating the PCBs including the passive components embedded therein, achieves substitution of conventional chip resistors or chip capacitors by mounting the passive components, such as the resistors or capacitors, on an external surface of a PCB or in an internal layer of the PCB according to a process employing a formed dielectric or resistive material, such as a ceramic, carbon, or metal oxide powder, which may be mixed into an epoxy paste.
On the whole, the technology of fabricating a PCB including a capacitor or resistor embedded therein may be classified into three methods. A first method of fabricating a polymer thick film type of circuit elements applies a filled polymer paste, which is then thermally hardened (dried or cured) to fabricate a device. In this method, after the filled device, typically a capacitor, is applied on an internal layer of a PCB and dried, a copper paste is printed on the resulting PCB and dried so that electrodes are formed, thereby making an embedded device.
A second method is to apply a filled photosensitive resin on a PCB to fabricate a discrete type of device. The photosensitive resin which contains a filler powder is applied on the printed circuit board (PCB), and a copper foil is laminated on the resulting PCB to form upper and lower electrodes, a circuit pattern is formed, and the photosensitive resin is etched to fabricate the discrete type of device.
A third method provides an electrochemically deposited film or film precursor layer on a substrate, typically a copper trace or pad, to form a functional film thereon, typically a dielectric layer of a capacitor. The electrochemical process, however, is associated with a corrosive and hostile environment to the underlying conductor, e.g., copper, leading to suboptimal properties of the resulting device. In particular, the environment during the deposition or a post-deposition conversion process may be oxidizing, resulting in corrosion of the copper or other metal and reaction with solutes, and hydrolysis of the aqueous solvent and generation of hydrogen, resulting in embrittlement of the metal and/or separation of the conductor from its substrate. The hydrolysis also results in energy inefficiency.
In some cases, an electrochemical process leads to a protective coating which is non-conductive, and is thus self-limiting with respect to the conduct of an electrochemical process through the coating. Therefore, such processes are typically limited to thin protecting coatings (e.g., an anodization process to produce a protective oxide). Formation of coatings of arbitrary thickness is difficult, and the coatings may have properties which are limited by the process conditions, and not as an independently controlled factor over a range of thicknesses.
Therefore, the electrochemical formation of coatings on conductive surfaces are disfavored as compared to deposited thick films for the fabrication of embedded devices.
Studies known in the art have demonstrated that insertion of an additional layer having a capacitance or resistance or other desirable property in an internal layer of a PCB so as to substitute for a decoupling capacitor or termination resistor, for example, conventionally mounted on a surface of a PCB, can have advantageous properties, often owing to a reduction in parasitic effects and inductance.